If your fall included a scrape, then I would say that this 'blistering' is your body's way of protecting the area while new skin grows underneath. Do not drain the blisters. Leave them alone and clean gently. Blessings - Blu
As you fell there must have been friction caused and that is why the blisters have formed.
Medelp do have a video showing how to get the fluid out of a blister by using a sterilised pin or needle and then applying some antiseptic cream to it. Some people prefer to leave them alone.
Apply an ice-pack to the knee. Another good alternative is to apply a lot of set yogurt and wrap it with a cloth. Do this when you are resting and before bedtime. The yogurt cools the area and draws out the inflammation. I did this to my sprained ankle when I couldn't walk on it and by the next day it was fine. I couldn't believe how quickly it recovered and but it down to having it strapped with lots of yogurt.
Applying yogurt to sore and itchy genitals and to sunburn also works.
Best wishes.